Spherical corundum has been known for approximately 75 years, and has been commercially manufactured for approximately 30 years. U.S. Pat. No. 1,871,792 and U.S. Pat. No. 1,871,793 describe blowing of a pouring stream of liquid corundum under reducing conditions, using compressed air or steam. Hollow corundum spheres having a diameter of approximately 0 to 5 mm are obtained. All significant steps of the manufacturing process, still in use today, have previously been described in the above-referenced U.S. patents.
Hollow spherical corundum is primarily used as a fire-resistant material because it has very low thermal conductivity on account of the air spaces incorporated in the spheres. Hollow spherical corundum is also used as a filter medium for hot, chemically corrosive gases and liquids.
In addition to hollow spheres, however, more compact spheres having a high mechanical resistance to pressure, impact, and abrasion are also of technical interest.
The physical properties of spherical corundum are characterized chiefly by the sphere size (spherical diameter), spherical wall thickness, and the crystallite size of the aluminum oxide primary crystals from which the corundum sphere is produced.
Depending on the use, quite different requirements are imposed on the properties of spherical corundum. Thus, numerous attempts have been described in the past for influencing the properties of spherical corundum by varying the raw material used or additives in the melt, or by modifying the casting technique (pouring quantity, manipulation of the pouring stream, air or steam pressure, nozzle shape, etc.).
The manufacture of compact aluminum oxide spheres by blowing is described in U.S. Pat. No. 2,261,639, in which compact spheres are obtained by addition of 1 to 10% sodium oxide in the melt and subsequent blowing. U.S. Pat. No. 2,340,194 describes the addition of 1 to 1.5% titanium oxide in the melt, resulting in pressure-resistant hollow spheres having relatively thick walls. However, the aluminum oxide spheres manufactured according to this method have the disadvantage that they contain foreign ions (sodium, titanium) as impurities. This may be disadvantageously noticeable when these materials are used as shot-blasting abrasives because the foreign ions can result in undesired reactions with the surface to be treated. In the case of sodium oxide as an impurity, sodium aluminate may be formed which has only a slight abrasive effect.
EP 1 157 077 describes the manufacture of polycrystalline abrasive grains, with the liquid corundum being poured and the cooling thereof being assisted by dispersion of the fused aluminum oxide into fine drops, using ultrasound. Compact particles having an average diameter of less than 1 mm are obtained in this manner. Aside from the fact that the particles are relatively small, which limits their area of application, the method is comparatively complicated, and the use of ultrasound is not suitable for mass production.
Thus, there is a continued need for more compact aluminum oxide spheres which can be manufactured by an easily performed, effective method.